Historically, integrated-circuit (IC) fabrication has been based on planar processing methods in which a stack of material layers are sequentially deposited and patterned on a semiconductor substrate. A typical material stack includes several dielectric layers (e.g., oxides, nitrides, etc.) as well as several metal layers for establishing electrical contact to individual devices, as well as interconnecting those devices to define electrical circuits.
Proper formation of the patterned metal layers is critical for successful device fabrication. In conventional planar processing, each metal layer is deposited, in blanket form, using a physical vapor deposition (PVD) process, such as thermal or e-beam evaporation or magnetron sputtering.
In an evaporation process, one (or a few) substrates are held in an evacuated reaction chamber that also holds a target of the desired metal. Heat or e-beam energy is used to melt a portion of the target. Since the chamber is under vacuum, the molten material quickly vaporizes in the low-pressure atmosphere. The vapor travels from the target to exposed surfaces that are within line of sight of the target, where it condenses to form a continuous layer. Evaporative PVD is limited to the deposition single-constituent metals having a relatively low melting point, such as aluminum and gold.
During sputter deposition, one or more substrates are collocated with a material target in a reaction chamber having a low-pressure atmosphere of inert gas. Material is liberated from the target by bombarding it with ions or atoms energized from the inert gas. The liberated material travels ballistically and deposits onto exposed surfaces, including the surface of the substrate(s). Sputtering enables deposition of metals having higher melting points (e.g., refractory metals such as tungsten, titanium-tungsten, etc.), as well as alloys or other composite materials whose deposited composition is close to that of the original target material. Unfortunately, sputter deposition is more complex than evaporation and is typically incompatible with some methods for patterning the resultant layer.
Once a blanket layer has been deposited, it is patterned by removing unwanted portions of the layer (referred to as subtractive patterning) before the deposition of the next layer. In most cases, the unwanted regions of the metal layer are removed by forming a photoresist mask on the layer that the undesired areas of the metal to a subsequent etch process. In some cases, the metal layer is patterned using a lift-off process wherein the layer is deposited over a pre-patterned layer of photoresist having openings where metal is desired. After layer deposition, the photoresist is dissolved away thereby “lifting” any metal on it and removing it from the substrate. Due to the high-energy nature of the ejected target material in sputter deposition, lift-off techniques cannot normally be used to pattern a sputtered layer.
While conventional subtractive patterning enables excellent pattern fidelity and small feature sizes, it is typically characterized by high material waste. In addition, the need to locate the substrate within an evacuated reaction chamber during evaporative or sputter deposition limits the size of the substrate and adds system cost and complexity. Further, wafer-based processing is inherently low throughput as compared to many modern manufacturing approaches, such as reel-to-reel manufacturing (a.k.a., roll-to-roll manufacturing). Still further, advancements in implantable biosensors, wearable sensors, as well as other applications, is driving a growth in the need for flexible electronics; however, conventional planar processing is normally limited to rigid substrates and not compatible with the polymer films normally used for flexible electronics substrates. Finally, the evaporative and sputtering processes used for metal deposition in planar processing approaches are characterized by high heat that can be damaging to many flexible-substrate materials.
As a result, there has been a concerted effort to develop a direct-writing process for depositing conductive material only where it is desired. To date, colloidal ink-based printing methods, such as ink-jet printing, screen printing, and aerosol printing, are the most promising additive approaches for producing directly written conductive layers.
A common feature of colloidal ink-based printing methods is the need for conductive inks, which typically include organic-ligand stabilized dispersions of metal nanoparticles. Such approaches are attractive because they can be carried out at temperatures near room temperature and substantially at atmospheric pressure.
Unfortunately, colloidal inks have many drawbacks. For example, the inks themselves can be expensive due to the number of processing steps associated with synthesis, dispersion, purification, and concentration. In addition, the variety of available materials suitable for printable inks is low. Silver is the most common commercially available ink, while other metals that are commonly used in electronic devices, such as Au, are not readily available. Further, the organic capping agents that are used to stabilize the suspended metal particles can be difficult to remove after deposition. This can lead to poor conductivity and compromised mechanical integrity and high annealing temperatures are normally required, which can limit the use of many polymers and other temperature-sensitive substrates (e.g., CMOS chips containing electronic devices). Still further, adhesion can also be a significant issue, especially for flexible-substrate applications and shelf life of colloidal inks is typically short due to issues related to maintaining a uniform suspension.
In order to avoid the drawbacks associated with colloidal inks, “ink-less” approaches have been under development. For example, e-beam exposure of films comprising metal-salt and polymer has been used to give rise to in-situ reduction and synthesis of nanoparticles in polymer films. Such ink-less processes avoid multiple processing steps and have the potential to limit organic impurities. Electron-beam-based approaches are also superior to colloidal ink-based printing in terms of pattern resolution; however, they require high vacuum, complex equipment, and must be deposited on conductive substrates. In addition, they are normally characterized by relatively low throughput. These issues remain as critical obstacles to large-scale production and wide-scale adoption of colloidal inks for direct-write electronics.
As a result, the need for a direct-write, additive process for producing patterned material layers on a wide variety of flexible and rigid substrates, which can be performed in substantially ambient conditions remains unfulfilled.